Providing fluids with the proper frictional characteristics for power transmitting devices is the responsibility of the fluid formulator. There are three aspects to producing fluids with the proper frictional characteristics. The first is having the correct friction break-in at the moment of initial fill of the device. The second is having appropriate friction after a short break-in period and the third is maintaining those frictional characteristics for long periods of time. This third characteristic is often referred to as friction durability. The present invention is concerned with the first of these properties, i.e., friction break-in.
Formulating power transmission fluids to very exacting friction requirements is a very difficult process. In the case of fluids used for initial fill, i.e. factory fill, of transmissions this problem is made more difficult because all of the components of the system, fluid, friction material and steel running surface are new. That is, they have experienced no conditioning under the conditions of operation of the system. Therefore, for the first several hours up to the first thousand miles of operation, the frictional characteristics of the system are constantly changing. Typically, fluid formulation is done to provide the ideal frictional characteristics in the "broken-in" or aged system.
The continuing search for methods to improve overall vehicle fuel economy has identified the torque converter, or fluid coupling, used between the engine and automatic transmission, as a relatively large source of energy loss. Since the torque converter is a fluid coupling it is not as efficient as a solid disk type clutch. At any set of operating conditions (e.g., engine speed, throttle position, ground speed, or transmission gear ratio), there is a relative speed difference between the driving and driven members of the torque converter. This relative speed differential represents lost energy which is dissipated from the torque converter as heat.
One method of improving overall vehicle fuel economy used by transmission manufacturers is to incorporate into the torque converter a clutch mechanism capable of "locking" the torque converter. "Locking" refers to eliminating relative motion between the driving and driven members of the torque converter so that no energy is lost in the fluid coupling. These "locking" or "lock-up" clutches are very effective at capturing lost energy at high road speeds. However, when lock-up clutches are used at low road speeds vehicle operation is rough and engine vibration is transmitted through the drive train. Rough operation and engine vibration are not acceptable to drivers.
The higher the percentage of time that the vehicle can be operated with the torque converter clutch engaged, the more fuel efficient the vehicle becomes. A second generation of torque converter clutches have been developed which operate in a "slipping" or "continuously sliding mode". These devices have a number of names, but are commonly referred to as continuously slipping torque converter clutches. The difference between these devices and lock-up clutches is that they allow some relative motion between the driving and driven members of the torque converter, normally a relative speed of 10 to 100 rpm. This slow rate of slipping allows for improved vehicle performance as the slipping clutch acts as a vibration damper. Whereas the "lock-up" type clutch could only be used at road speeds above approximately 50 mph, the "slipping" type clutches can be used at speeds as low as 25 mph, thereby capturing significantly more lost energy. It is this feature that makes these devices very attractive to vehicle manufacturers.
However, continuously slipping torque converter clutches impose very exacting friction requirements on automatic transmission fluids (ATF's) used with them. The fluid must have a very good friction versus velocity relationship. The parameter commonly used to quantify a fluid's friction versus velocity relationship is the change of friction with sliding speed, .DELTA..mu./.DELTA.v. For the continuously slipping torque converter clutch to operate properly friction must always increase with increasing speed, a positive .DELTA..mu./.DELTA.v. If friction decreases with increasing speed, a negative .DELTA..mu./.DELTA.v, then a self-exciting vibrational state can be set up in the driveline. This phenomenon is commonly called "stick-slip" or "dynamic frictional vibration" and manifests itself as "shudder" or low speed vibration in the vehicle. Clutch shudder is very objectionable to the driver.
A fluid which allows the vehicle to operate without vibration or shudder is said to have good "anti-shudder" characteristics. Not only must the fluid have an excellent friction versus velocity relationship when it is new, it must retain those frictional characteristics over the lifetime of the fluid, which can be the lifetime of the transmission. The longevity of the anti-shudder performance in the vehicle is commonly referred to as "anti-shudder durability".
When the continuously slipping torque converter mechanism is installed in the new vehicle and the vehicle operated for the first several minutes, up to the first several hundred miles, the device must operate as designed, and be free from shudder. During this break-in, or run-in, many changes are occurring in the components of the continuously slipping torque converter clutch. The steel running surface is wearing, friction material is being worn, resins in the friction material, e.g., a friction disk, are further cured and fluid is beginning to age at the high operating temperatures. All of these changes affect the overall frictional characteristics of the system. In order for the continuously slipping clutch to operate properly, the correct friction versus velocity relationship must be established and maintained all throughout this break-in period, and beyond. Shudder which occurs during the break-in period is commonly referred to as "green shudder".
What the present inventors have discovered is that by employing an oil-soluble phosphorus compound, an ashless dispersant and a long chain alkyl primary amine, power transmitting fluids with excellent break-in friction characteristics can be produced.